Noise contaminates image data in the image input systems or image transmission systems of image input devices such as digital cameras or the like having CCD or other imaging elements, or image output devices such as displays that display images input from these types of image input devices. This type of noise is generally reduced by image processing in the device. Conventional methods of reducing noise contamination in such image input devices or image output devices will now be described.
The simplest and most general way to reduce noise is a technique known as low-pass filtering, in which a processing window consisting of a plurality of pixels centered on the pixel to be processed (referred to below as the pixel of interest) is defined, and a weighted sum of the pixel values of the pixel of interest and neighboring pixels in the processing window is taken. When this method is used, however, there is the problem that the low-pass filtering process also affects meaningful edges in the image that are not noise, lowering the resolution of the image.
Another proposed technique is to apply a Prewitt filter or a Sobel filter to the processing window to detect edges, and reduce the strength of the low-pass filter when the pixel of interest is found to be part of a meaningful edge in the image, or to apply the filter in the direction of an edge present nearby, thereby reducing noise with less loss of resolution (see, for example, Patent Reference 1).